The present invention relates to optical communications networks. More particularly, it concerns systems and methods for effecting reliable and accurate frequency control within optical communications networks.
Various information multiplexing arrangements, including time division multiplexing (TDM) and frequency division multiplexing (FDM), can be used for transferring information within an optical communications network. With time division multiplexing, information is transmitted in predetermined time slots relative to a recurring system timing cycle, and with frequency division multiplexing, information is transmitted simultaneously on frequency distinct carriers or channels. Accurate timing devices are required in time division multiplexing systems while accurate frequency determining and controlling devices are required in frequency division multiplexing systems to create and maintain many independent frequency distinct information channels.
In the suboptical portion of the electromagnetic spectrum, accurate and highly reliable frequency determining devices have been developed to allow the creation of sophisticated frequency division multiplexing systems at reasonable cost. At optical frequencies, however, corresponding frequency determining devices have not yet become available. In optical systems, various devices are known for providing a time invariant frequency output that can be used as a frequency reference. In general, such devices are too expensive or complicated to allow widespread implementation in an optical communication system. For example, the absorption or emission spectra of certain plasmas such as sodium or cesium can be utilized as a time invariant frequency reference. Additionally, certain gas lasers such as the He-Ne laser can likewise provide a time invariant output at a known frequency. These devices while providing the desired output pose a number of drawbacks from a practical standpoint. In general a highly stable output frequency is available at one or a few frequencies, which frequencies may or may not be desirable for establishing communications channels. In addition the cost of these devices can be inconsistent with inexpensive optical communications networks having many terminal devices especially where each terminal device must include a local frequency reference.